Further insights into Allan-Herndon-Dudley syndrome: characterization of two genetic variants in SLC16A2 gene

Genetics variants in SLC16A2 gene encoding for the monocarboxylate transporter 8 (MCT8) cause a severe X-linked intellectual deficit and neurological impairment known as Allan-Herndon-Dudley syndrome (AHDS). MCT8 promotes cellular uptake and efflux of thyroid hormone and its mutations provoke elevated serum T3 levels in children. Iodothyronine deiodinases (DIO) 1 and 2 are implicated in the conversion of T4 into biologically active T3, while DIO3 converts T4 into the inactive hormone reverse T3 (rT3). Active T3 and retinoid X receptors (RXR) can form heterodimer complexes which bind to hormone response elements (HREs) that leads to activate or repress transcription. Our aim is to investigate the impact of MCT8 mutations on the pathogenetic mechanisms of AHDS. Fibroblasts were obtained from skin biopsies of 2 AHDS and matched controls. To evaluate both MCT8 and thyroid hormone signaling pathway related genes expression, RNA was extracted with TRIzoLTM and assessed by Real-Time PCR. Protein expression was valuated via western blot and immunofluorescence. MTT assay was used to compare cell viability. Live and dead assay was used to discriminate live and dead populations. Lipids were detected via oil red o staining. MTT assay demonstrated a reduced cell viability as consequence of mutations in SLC16A2. We report that SLC16A2 RNA expression in AHDS patients was extremely reduced in comparison with total RNA from healthy controls. Additionally, DIO2, progastricsin, HR and KLF9 RNA expression resulted upregulated, whilst DIO1, DIO2-AS1, DIO3 and TH were downregulated influencing T3 cell entrance. Myelin related genes were significatively reduced. The lipid staining revealed an increasing presence of lipid droplets in AHDS patients. Taken together, our preliminary data emphasize an impairment in AHDS fibroblasts in relation to mutations in MCT8 transporter, increasing our understanding in the pathogenic mechanism of mutation in two patients affected by AHDS.